1. Field of the Invention
This invention is concerned with the overcurrent protection of converter circuits and, in particular, in limiting the output current in a selected output of a multiple output converter circuit. In particular, it is concerned with combining an action of a primary current limiting circuit which limits the total aggregate output current of the converter with a secondary current limit circuit to independently limit the current output at selected outputs.
2. Prior Art
A common current limiting arrangement in DC-to-DC converters is located on the primary side of the power transformer and operates by sensing the peak current flowing through the primary winding of the power transformer. This design is widely used and has the advantage of having low cost and being relatively high in efficiency.
A primary current limit control arrangement such as described above is disclosed in U.S. Pat. No. 3,859,586 issued to J. C. Wadlington on Jan. 7, 1975, and assigned to the same assignee as this application. In the primary current limit circuit disclosed therein the current in the primary winding of the power transformer is sensed and the duty cycle of the converter switching devices is controlled to limit the maximum current. In one particular arrangement shown a primary current limit circuit is disclosed to accommodate a converter having multiple outputs. In a multiple output converter utilizing primary current limiting control, a single output current may be at a very high level above its allowable current limit while the primary current being sensed is still within limits. This limitation of primary current limiting in the Wadlington patent is overcome by utilizing foldback techniques in current limit control. In the current limit circuit disclosed therein a foldback response of the current limit circuit to a detected overcurrent reduces the total current output to less than the maximum current allowed for any one output. This arrangement, while a definite improvement, still permits the existence of an overcurrent at one output without automatically activating the overcurrent limit circuit.
An alternate approach to current limiting individual outputs of a multiple output converter is to use independent current limit circuits in each current output. However, this generally requires a separate power source or a tertiary winding on the transformer to supply power to operate the current limiting circuitry. The former arrangement is costly and the latter arrangement is not compatible with a converter having primary current limiting circuitry. This incompatibility arises because the voltage of the tertiary winding will track the output voltage to an unusable level during current limiting, hence, during current limiting the current limiting circuit itself is liable to become inoperative.